1. Field of the Invention
The present invention relates to a metal matrix composite formed by consolidating under heat and pressure a sandwich of alternating layers of a metal or a metal alloy, reinforcing fibers, and a bonding aid or brazing alloy. The present invention also relates to a method for keeping the metal matrix composite free of the forming mandrels used in a roll consolidating process to apply the heat and pressure.
2. Description of the Related Art
In order to enhance the mechanical properties of monolithic materials used, for example, in advanced aerospace systems, one approach has been to develop composite materials in which two dissimilar materials having different mechanical and physical properties are combined to form a product having mechanical and physical properties superior to the individual constituents. For example, metals having good ductility and strength can be combined with fibers having low ductility, higher strength and stiffness, and lower density to form a stiff, lightweight composite material.
An example of such a composite material comprises titanium, which has a high specific strength (i.e., tensile strength to density ratio) and excellent corrosion resistance, combined with alumina-based, boron-based, or silicon carbide-based fibers. Many other fiber systems exist, e.g., graphite, which could also be utilized depending on the application. There are a number of processes for consolidating Such composite materials including vacuum hot pressing and hot isostatic pressing.
Vacuum hot pressing involves stacking alternating layers of a metal matrix and fiber or layers of a fiber and matrix monotape and placing them in a vacuum hot press. After evacuation, a small positive mechanical pressure is applied via ceramic platens to hold the fibers in place. The temperature is gradually increased to a level wherein the metal matrix flows superplastically around the fibers under an increased pressure. This process, however, involves long heat cycle times, requires tight die tolerances, and uses large amounts of electrical power.
Hot isostatic pressing involves simultaneously subjecting a metal matrix and fiber composition to elevated temperatures and isostatic pressures. However, hot isostatic pressing involves the use of high gas pressure and thus requires very substantial vessels in order to manufacture large parts, resulting in very high equipment costs. Therefore, typically, only smaller metal matrix composite components are consolidated using this process due to the aforementioned equipment size and cost constraints.
The strict processing requirements, i.e., high temperatures and pressures and long processing times, associated with the vacuum hot pressing and hot isostatic pressing processes result in high production costs and complicated machinery and fixturing.
Another example of such a composite material comprises a thermoplastic combined with carbon-based fibers. Standard processes for consolidating these composite materials include autoclave processing or hot pressing. However, these processes, like the processes for consolidating metal matrix composites, involve long heat cycle times and require expensive tooling and equipment.